Thermally treated container wall

ABSTRACT

A method of fabricating a container wall having an integral opening device by providing a metal sheet having an adhesive material secured to one surface thereof. The adhesive secures a continuous protective layer to the metal sheet. The protective layer is selected from the group consisting of partially crystalline polyolefin, partially crystalline polyolefin-ethylene copolymers, polyethylene acrylate and ionomers of polyolefin. Scoring the sheet to define a removable sector therein while preventing fracture of the adhesively secured protective layer. Subsequently heating the composite metal sheet to a temperature of about 275* to 375* F for a period of about 0.2 second to 4 minutes and preferably about 300* to 350* F for about 0.5 to 3 minutes. The protective layer is stress relieved by this thermal treatment which substantially completely eliminates microvoids established in the protective layer during the scoring operation. Effecting the heating operation in such a fashion as to improve adhesive bonding of the protective layer, resist diaphragming of the protective layer and reduce feathering of the protective layer along the line of severance when the removable sector is fractured. A container wall structure made by the process of this invention including the adhesive being selected from the group consisting of epoxy, polyester and polyurethane. The protective layer having a crystallinity in excess of about 50 percent. The adhesive bond between the protective layer and the metal sheet removable sector being of greater strength than the shear strength of the protective layer in regions underlying the score line to prevent substantial diaphragming and to limit feathering to less than about one-thirty-second inch.

United States Patent [1 1 Gayner et al.

[ Oct. 2, 1973 THERMALLY TREATED CONTAINER WALL [75] Inventors: HerbertGayner, Monroeville; David A. Smith, Penn Hills, both of Pa.

[73] Assignee: Aluminum Company of America, Pittsburgh, Pa.

[22] Filed: Oct. 19,197]

[21] Appl. No.: 190,480

Primary Examiner-George T. Hall Attorney-Arnold B. Silverman [57]ABSTRACT A method of fabricating a container wall having an integralopening device by providing a metal sheet having an adhesive materialsecured to one surface thereof. The adhesive secures a continuousprotective layer to the metal sheet. The protective layer is selectedfrom the group consisting of partially crystalline polyolefin, partiallycrystalline polyolefin-e'thylene copolymers, polyethylene acrylate andionomers of polyolefin. Scoring the sheet to define a removable sectortherein while preventing fracture of the adhesively secured protectivelayer. Subsequently heating the composite metal sheet to a temperatureof about 275 to 375 F for a period of about 0.2 second to 4 minutes andpreferably about 300 to 350 F for about 0.5 to 3 minutes. The protectivelayer is stress relieved by this thermal treatment which substantiallycompletely eliminates mi crovoids established in the protective layerduring the scoring operation. Effecting the heating operation in such afashion as to improve adhesive bonding of the protective layer, resistdiaphragming of the protective layer and reduce feathering of theprotective layer along the line of severance when the removable sectoris fractured.

A container wall structure made by the process of this inventionincluding the adhesive being selected from the group consisting ofepoxy, polyester and polyurethane. The protective layer having acrystallinity in excess of about 50 percent. The adhesive bond betweenthe protective layer and the metal sheet removable sector being ofgreater strength than the shear strength of theprotective layer inregions underlying the score line to prevent substantial diaphragmingand to limit feathering to less than about one-thirty-second inch.

4 Claims, 9 Drawing Figures Pmmmncm 3.762.598

SHEET 1 OF 2 FIG. 4.

PATENTED 2 m 3.762.598

SHEET 2 0F 2 FIG. 5. FIG. 50.

FIG. 6.

FIG. 7

THERMALLY TREATED CONTAINER WALL BACKGROUND OF THE INVENTION 1. Field ofthe Invention This invention relates to a thermal treatment process forlaminated container walls having integral opening devices and theproduct thereof. More specifically, this invention relates to apost-heating process which improves the opening characteristics of thelaminated container wall and produces improved barrier properties.

2. Description of the Prior Art It has been conventional to provideforms of container wall structures which have a score line definedremovable sector to which a pull tab has been secured by means of arivet formed integrally with the container wall. Such container wallsoffer the consumer the advantage of not requiring the use of a separatecontainer opener. In addition, the removable sector is generallyprovided in such a fashion as to permit relative ease of opening,particularly for a consumer obtaining the mechanical lever advantageprovided by a lever form of pull tab.

In one of the conventional methods of fabricating such container endwalls, the undersurface of the end is coated with a suitable protectivecoating prior to conversion of the wall to provide the integral openingdevice. During scoring and rivet forming the container wall element issubjected to extreme compressively applied impact forces which cause theconventional coating on the undersurface to fracture. The protectivecoating must have its integrity maintained in order to avoid corrosionhazards during use of the container wall. Such hazards can be producedby corrosive attack on the metal by the container contents and, also,galvanic corrosion where bimetallic container assemblies are employed.It is, therefore, necessary in such conventional fabricating systems toprovide a supplemental process step in which the damaged coating isrepaired. This approach has involved the need to employ extra equipmentand an extra process step to apply additional material which fills thevoids and repairs the coating.

It has also been known to provide certain laminated container wallshaving opening devices and employing specific protective materials whichare adapted to withstand the extreme forces applied during scoring andrivet formation. Such materials eliminate the need for repair coating.Such approaches are disclosed in U.S. Pat. Ser. No. 825,164, now U.S.Pat. No. 3,632,461, entitled "Laminated Container Wall Structure," andalso in U.S. Pat. Ser. No. 190,479, entitled Laminated Container Wall"and filed by applicants on even date herewith.

It has previously been suggested that thermal treatment may bebeneficial in healing physical cracks created through an enamel coatingduring the formation of container walls from enamel coated sheet. SeeU.S. Pat. No. 2,086,165. This disclosure did not involve a containerwall which was subjected to the severe forces of scoring and rivetforming which are encountered in forming a removable sector. Inaddition, it was concerned solely with a process which melts the enamelmaterial in order to provide flow of the same into the cracks createdduring shaping of the article.

With respect to the types of container walls disclosed in U.S. Pat. Ser.No. 825,164 now U.S. Pat. No.

3,632,461 and Ser. No. 190,479, described above, there have been certaininstances when the protective layer has diaphragmed in an undesirablefashion by partially losing its adhesive bond to the removable portionof the metal sheet. This can result in severance of the metal sheetremovable sector without severance or with incomplete severance of theunderlying protective layer. The result is a full opening in the metalsheet which has been restricted by the presence of an intact protectivelayer or an opening of smaller size in the underlying protective film.This same undesirable end result has occurred occasionally as a resultof excessive stretching of the underlying protective layer duringseverance of the removable sector which also results in the creation ofa restricted opening in the protective film.

There remains, therefore, a need for a laminated container wall whichwill not only provide an effective barrier which will survive the forcesapplied during scoring and rivet forming, but also will maintain aneffective adhesive bond with the metal sheet and provide im provedopening characteristics for the integral opening device.

SUMMARY OF THE INVENTION The thermal process and resultant product ofthis invention have filled the above-described need. The processinvolves providing a metal sheet having an adhesive material secured toone surface thereof and securing to the metal sheet a continuousprotective layer. The protective layer is preferably a film selectedfrom the group consisting of partially crystalline polyolefin, partiallycrystalline polyolefin-ethylene copolymers, polyethylene acrylate andionome'rs of polyolefin. The metal sheet is scored to define a removablesector therein, while preventing fracture of the adhesively securedprotective layer. Subsequently, the composite metal sheet is heated to atemperature of about 275 to 375 F for a period of about 0.2 second to 4minutes, preferably about 300 to 350 F for about 0.5 to 3 min utes. Thisthermal treatment serves to provide stress relief for the protectivelayer whichsubstantially completely eliminates microvoids established inthe protective layer during the scoring operation and any integral rivetforming operation.

The container wall structure of this invention has a metal sheetprovided with a removable sector defined by a score line. An adhesivelayer is secured to the undersurface of the metal sheet with theadhesive preferably selected from the group consisting of epoxy,polyester and polyurethane. A stress relieved thermally treatedcontinuous protective layer is secured to the undersurface of the metalsheet by means of the adhesive layer. The protective layer preferablyhas a crystallinity in excess of 50 percent and is bonded to the metalsheet removable sector in such a fashion that the bond therebetween hasa greater strength than the shear strength of the protective layer inregions underlying the score line. As a result, severance of the scoreline and displacement of the removable sector automatically establishesseverance of the underlying protective layer without substantialdiaphragming and without undesired excessive feathering.

It is an object of this invention to provide a method of thermallytreating composite container walls having integral opening devices insuch a fashion as to improve adhesive bonding and barrier properties aswell as facilitating opening of the container wall without undesiredobstructions being created by the protective layer.

It is another object of this invention to provide such a method ofthermal treatment of the container wall which is employed after scoringand/or rivet forming in order to stress relieve the protective layer insuch a fashion as to substantially completely eliminate unde sirablemicrovoids established in the protective layer during such scoringand/or rivet forming.

It is another object of this invention to provide such a'process andproduct which are adapted to be employed economically in connection withthe manufacture of a wide variety of types of container walls composedof a wide range of materials.

It is yet another object of the present invention to provide such athermal post-treatment process wherein the physical position of theamorphous and crystalline portions of the protective layer is restoredto a position approximating the state prior to conversion to create theintegral opening device.

These and other objects of the invention will be more fully understoodfrom the following description of the invention, on reference to theillustrations appended thereto.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of a type oflaminated container walls of this invention.

FIG. 2 is a fragmentary cross sectional illustration of a form oflaminated wall of this invention showing a scored portion thereof takenthrough 22 of FIG. 1.

FIG. 3 is a fragmentary cross sectional illustration taken through 3-3of FIG. 1 showing an integral rivet portion of a laminated containerwall of this invention but with a slightly modified form of tab profile.

FIG. 4 is a fragmentary cross sectional illustration showing the scoringprocess.

FIG. 5 is a fragmentary cross sectional illustration of a container wallshowing undesirable microvoids in the protective film region underlyingthe score line.

FIG. 5a is an enlarged fragmentary cross sectional illustration of aportion of the protective layer shown in FIG. 5.

FIG. 6 shows a fragmentary cross sectional illustra- .tion of acontainer wall having undesirable diaphragming characteristics.

FIG. 7 is a fragmentary plan view of a container wall of this inventionshown after the removable sector has been severed and withdrawn.

FIG. 8 is a fragmentary cross sectional illustration taken through 88 ofFIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As used herein the termcontainer wall" and words of similar import shall refer to wall portionsof containers which are provided with opening devices and shall includecan ends, jar and bottle closures and other forms of container portionsmeeting these specifications.

Referring now more specifically to FIGS. 1 and 2, there is shown a canend having a top panel 2 and an annular chuck wall 4 which terminates ina radially outwardly directed flange 6. The top panel 2 is provided witha score line 8 which defines a removable sector 10. The term removablesector" and words of similar import shall refer to sectors which arecompletely severable from the remainder of the container wall, as wellas sectors which are adapted to be partially severed and displacedsufficiently to permit access to the container contents withoutcompletely separating the same from the container wall. While forconvenience of reference herein the term score line will be used in thesingular, it will be appreciated that the removable sector 10 may bedefined by one or more score lines or weakened lines provided by anotherfashion so long as residual metal is retained in the panel at thelocation of the score or weakened lines.

A pull tab 14 has an opening 16 adapted to facilitate manual engagementthereof at one end and a securing portion 18 through which fasteningmeans, such as integrally formed rivet 20, may pass in securing the pulltab 14 to the top panel 2.

In initiating score line fracture, the pull tab is grasped in the regionof opening 16 and raised in order to cause the radially innermostportion of the removable portion 10 to be severed through fracture ofscore line 8. Sequential severance of the remainder of score line 8permits complete separation of removable sector 10. In the form shown inFIG. I, the top panel 2 has a pair of elongated hollow stiffening ribs12 disposed on opposite sides of the tear strip 10. The type ofremovable sector shown in FIG. 1 is that which would generally beemployed with pourable container contents. The invention, however, isequally applicable to removable sectors which encompass substantiallyall of the top panel 2 including the so-called full panel pull outcontainer walls.

Referring now to FIGS. 2 and 3, there is shown a portion of thecontainer wall illustrated in FIG. 1. The container wall has a metalsheet or panel portion 22 which is provided with an adhesive layer 24. Aprotective layer 26, which may be applied as either a coating or filmwith the latter being preferred, is continuous and is preferablycontinuously bonded to the metal sheet by means of adhesive 24.

The metal sheet 22 is preferably composed of aluminum or steel, as wellas alloys thereof. In a typical structure an aluminum-magnesium alloy ina thickness of about 0.008 to 0.0135 inch in the unscored portions isemployed. The residual material 28 underlying the score line 8 wouldhave a metal thickness of about 0.0025 to 0.0045 inch. For purposes ofsimplicity of description the term metal sheet as employed herein willbe used to refer to both the individual sheet elements out of which anindividual container wall will be formed and also to the parent stockout of which a number of individual container walls may be formed. Whilein general it will be most convenient to establish the compositeadhesively bonded material and then sever individual container wallblanks which are substantially scored, riveted and converted tocontainer walls, this preferance is in no fashion critical to thepractice of the present invention.

The adhesive layer 24 may conveniently be applied as a coating. Theadhesive material 24 is preferably applied in an amount of about 1.0 to5.0 mg/in with about 1.0 to 3.0 mg/in being the preferred amount. Theadhesive is preferably selected from the group consisting of epoxy,polyester and polyurethane. It will be appreciated that variousadditives, catalysts and modifying agents may be employed with theadhesives, should the same be desired. An epoxy modified polyurethaneadhesive employed with a conventional catalyst, for example, has beenfound to be effective.

The protective layer 26 is preferably continuous and is preferablysubstantially continuously bonded to the metal sheet 22 by means of theadhesive 24. The protective' material 26 is preferably selected from thel group consisting of partially crystalline polyolefin, partiallycrystalline polyolefin-ethylene copolymers, polyethylene acrylate andionomers of polyolefin. Among the specifically preferred materials arepolypropylene and a polypropylene-ethylene copolymer wherein theethylene content is about 2 to percent, by weight, with an ethylenecontent of about 2 to 5 percent, by weight, being preferred. Theprotective material will preferably have a crystallinity of about 50 to60 percent, with about 50 to 40 percent being amorphous. The protectivelayer 26 is preferably applied as a film. Generally the layer 26 willhave a thickness of about 1 to 4 mils with about 2.5 to 3.5 mils beingpreferred for maximum performance at the lowest material cost.

Referring now more specifically to FIG. 3 there is shown a portion ofthe top panel 2 of FIG. 1 including the integral rivet and the pull tab14. It is noted that the rivet 20 is provided with a transverse wall 30and rivet sidewall 32. Thesidewall 32 passes through an opening in thepull tab 14 andthe enlarged head portion provided by transverse wall 30and the upper portion of sidewall 32 serve to complete the mechanicaljoint and resist relative separating movement of the pull tab 14 and toppanel 2. It is noted that in the form shown the transverse wall 30 has athickness which is slightly less than the thickness of the top panel 2and substantial deformation of top panel 2 has been effected in thelocal region where the rivet 20 appears. While the rivet may be formedby a number of conventional rivet forming techniques, it will beappreciated that the barrier properties of the protective layer 26 willbe maintained in this region of severe mechanical forming which has beensubjected to the application of substantial forces.

Referring now to FIG- 4, there is shown a pairof complementary toolsemployed toeffect scoring in the top panel 2. An underlying anvil 36having a flat upper surface cooperates with an overlying indenter tool38 to establish score line 8, while retaining score line residualmaterial 28. In practice the score line is established by creating arelative closing movement between the indentertool 38 and anvil 36 insuch a fashion as to provide a high impact force to the localregion stobe scored. This force may frequently create a pressure of about 100,000to 300,000 pounds per square inch. The score line 8 is created byseverance and physical dislocation of portions of the panel and/orcompressively established flow of metal away from the score line region.In any event, it is desired that the final product have an intactprotective layer 26 effectively adhesively bonded tothe metal sheet 22,even in regions underlying the score line 8.

It has been found that although the laminate materials employed in thecontainer walls described herein will withstand the rivet forming andscoring operations without the creation of mechanical voids or cracksextending completely through the protective layer 26 and also withoutexcessive thinning thereof, some changes do occur. It is believed thatthese changes result from a molecular change withinthe material out ofwhich the protective layer 26 is composed. As the impact forces areapplied, it is believed that the polymer chains are distorted as aresult of the crystalline portions of the polymer being more rigid andmoving more slowly than the amorphous portions. This results in astructural distortion of the polymer chains with the stretchingproducing microscopic voids within the structure.

Referring now to FIGS. 5 and 5a, there is shown the score line 8 and anunderlying portion of the protective material 26 which is illustrated ashaving a plurality of micrscopic voids 42 (shown in somewhat distortedenlarged fashion for clarity of illustration). It is believed that thesevoids will in most cases contain a vacuum. Referring to FIG. 5a, it isseen that in the region designated A, the protective layer 26 has athickness T which amounts to the full web thickness of the protectivelayer 26. This full thickness T serves as a barrier to passage ofpotentially corrosive materials through the layer. As the penetration ofa given potentially corrosive material through protective layers 26 isrelated to the thickness of the material, region A provides maximumresistance to such penetration. It is seen that region B has microvoids42 which reduce the effective thickness of the protective layer 26 andtherefore provide local reductions in barrier effectiveness in regionsunderlying the score line 8 as well as regions adjacent to the areas ofheavy force application. Thus, although the microvoids 42 do not providephysical openings in the barrier material which would permit directcontact with the undersurface of metal sheet 22 they do reduce theeffective barrier thickness in these regions. They serve to create anundesired path through which potentially corrosive materials may pass inorder to reach metal sheet 22.

It has been found that the thermal process of this inventionextinguishes substantially all of the undesired microvoids 42 andthereby improves the barrier characteristics of the protective layer. Inaddition, advantages in improved adhesion and opening characteristics ofthe container wall, which will be described below, are effected. Thethermal treatment process of this invention involves post-heating of thecontainer wall, preferably after both scoring and rivet information havebeen effected and the container wall is converted to its finalstructural form. Heating is effected at about 275 to 375 F for a periodof about 0.2 second to 4 minutes and preferably is effected at about 300to 350 F for about 0.5 to 3 minutes. In selecting a time and temperaturewithin these ranges one may balance the added benefits obtained withinthe preferred range with the desired final properties and economiesinvolved in high speed fabrication. While the most effective thermalposttreatment is obtained within the preferred range, definite benefitsof the invention may be obtained within the broader recited range. Oneof the most practical means of post-heating involves the use ofinduction heating. Induction heating facilitates the use of reducedheating periods and corresponding increases in production rate.

Heating effects a thermally induced stress relief which substantiallycompletely eliminates the microvoids established in the protective layer26 resulting from the molecular structural distortion which occursduring scoring, rivet forming and possibly during wall forming. Thermaltreatment within this range is believed to produce no ultimate change inthe percentage of crystalline structure which exists in the protectivelayer 26. It serves to restore the structure to one approximating theoriginal molecular state and eliminate the undesired dislocations whichoccur as a result of the difference in rate of movement between thecrystalline and amorphous phases during scoring and rivet information.

It is believed that beginning at about l85 F the molecular structureexperiences crystallite loosening but no melting. At about 275 F somemelting is believed to occur with the smaller crystals liquefying first.Heating within the ranges of this invention to relieve stored energy issuch that a certain degree of melting is established and crystallinityis lost until the protective layer 26 is cooled. The melting is such,however, that no readily visible change in the general shape of layer 26and no substantial additional crystallization are induced. Thestructure, upon cooling, returns to what is essentially its originalstate with the original crystal structure being recreated and theamorphouscrystalline balance being maintained. It is generally best tooperate within the preferred temperature range, i.e., 300 to 350 F inorder to expedite the stress relief.

Referring to H6. 6, another form of potential failure of a laminatedwall construction will be considered. As used herein, the termdiaphragming shall refer to the creation of unbonded regions between themetal panel 22 and the protective layer 26 of sufficient size to providea void therebetween. More specifically, diaphragming will be employed torefer to such unbonded regions which occur between the undersurface ofthe removable sector and the underlying portion of the protective layer26. As is shown in FIG. 6, the protective layer 26 underlying theremovable sector 10 has an unadhered portion 44 which is disposed inspaced un' derlying relationship with respect to panel undersurfaceportion 46. While the adhesive 24 has been shown as adhered to unadheredportion 44, it might have adhered to removable sector 10 or partially toportion 44 and partially to sector 10. The manner in which the bondfails is not critical.

When diaphragming exists, it will be appreciated that severance of scoreline 8 in order to withdraw removable sector 10 will not automaticallyproduce severance of protective layer portion 44 in the desired manner.As a result, it may be necessary for the consumer to employ a separateimplement in order to sever the diaphragmed protective portion 44 andgain access to the container contents. In the preferred form of theinvention the bond between panel portion 46 and protective layer portion44 would be such that it would exceed the strength of the protectivelayer in regions underlying score line 8. Severance of score line 8 willautomatically produce severance of the underlying adhered protectivelayer portion 44 to create an opening in the protective layer 26 of thegeneral proportions of that of the removable sector automatically andwithout independent effort on the part of the consumer. It has beenfound that the thermal post-treatment process of this invention, withinthe temperatures recited above, substantially completely eliminatesdiaphragming and produces the desired improved adhesive bonding in thisregion and thereby facilitates automatic severance of the protectivelayer 26 during container opening.

Referring now to FIGS. 7 and 8, another advantage produced by thethermal treatment process of this invention will be considered. It hasbeen noticed that with respect to some laminated container wallstructurespwhich do not have diaphragming infirmities, an

additional problem may be encountered. ln effecting severance of theremovable sector 10 the underlying adhered portions of the protectivefilm will sever, but prior to severance will be subjected to permanentelongation which causes the protective layer 26 to define an accessopening which is smaller than the opening defined by the score line 8 inmetal panel 22.

As is shown in FIGS. 7 and 8, the container panel 2 has had theremovable sector 10 (not shown in this view) separated therefrom inorder to establish an opening 50in the metal panel 2, which is definedby the remaining portion 8' of the score line 8. The underlyingprotective material 26 has been stretched transversely inwardly andpartially upwardly with respect to opening 50 and defines a smalleropening 52. For conve nience of description herein the term feathering"shall be used to refer to the projection of the protective layer 26 intothe opening 50 in order to define a restricted opening 52. While acertain minor degree of feathering is not objectionable, any featheringwhich materially restricts opening 50 in such a fashion that restrictedopening 52 is substantially smaller is objectionable. As

- is shown in FlG. 7, the feathered projection 56 of protective layer 26extends into opening 50 by a distance C, when considered in plan, andrestricts opening 50 to that extent. It is generally desirable torestrict the average feathered projection 56 in such a fashion that thedimension C will be less than about one-sixteenth inch, with less thanabout one thirty-second inch being preferred. It has been found that thethermal posttreatment of this invention produces'a substantialimprovement in reducing the extent of feathering. Posttreatment at thetemperatures recited above will generally restrict the amount offeathering to the preferred limits.

In order to confirm the effectiveness of the thermal post-treatment ofthis invention in reducing or extinguishing diaphragming and feathering,two series of tests were performed in which identical container wallswere treated in the same fashion, but for the absence of thermaltreatment of some samples and the use of thermal treatment in others.This provided a direct basis for isolated comparison of the effect ofthermal treatment of this invention. The results of these tests arereported in the following examples.

EXAMPLE 1 Several groups of aluminum can ends made from analuminum-magnesium alloy in the extra hard H19 temper (505 lHl9) wereprovided with one of several types of laminated films. The films weresecured to the aluminum ends by means of an epoxy modified, polyurethaneadhesive. The ends in Groups A, C and E were provided with a 3 milpolypropylene film and the ends in Groups B and D were provided with a 3mil polypropylene-3 percent ethylene, by weight, copolymer film. Theends were scored in a conventional fashion to define removable sectorstherein. Integral rivets were formed in the ends in a conventionalfashion. The metal sheets out of which the ends in Groups A and B weremade were laminated at a laminating roll temperature of about 225 F,while the sheets out of which the Groups C, D and E ends were made werelaminated at a laminating roll temperature of about 300 to 320 F. Aportion of the ends of each group were post-heated at a laminating rolltemperature of about 325 F for 2 minutes, while the remainder of theends in each group Thermal Post-Treatment Grade No Fair Yes No Yes NoYes No Yes No Yes Feathering rt inch Excellent Fair 1/l6 inch Good 1/32inch Good 1/32 inch Excellent Good Good Fair Excellent 1/32 inch 1/32inch /64 inch These results show the superiority of the identicallaminated container walls employed with a thermal posttreatment ascompared with those without such thermal treatment. The only groups ofends rated excellent" were post-treated, while all groups of ends ratedfair were not post-treated. The only groups of ends wherein nofeathering was experienced were the postheated ends.

EXAMPLE 2 The tests performed in Example 1 were repeated with the use ofa slightly modified anvil die structure which had a large radius ofcurvature rather than being flat. The following results were obtained:

Thermal Post-Treatment Grade No Good Yes Good No Fair Yes Excellent NoFair Yes Excellent No Good Yes Excellent No Good Yes ExcellentFeathering 1/32 inch 1/32 inch 3/32 inch 3/32 inch 0 1/32 inch 0 UNIUUQOUJW 3/64 inch 0 These tests also confirm the general superiority ofthe thermally treated ends as compared with those which received nopost-treatment. All four of the groups of ends which received anexcellent rating and zero feathering belong to the post-treated groups,while those in the group which was not thermally post-heated were ratedeither as fair or good, with feathering ranging as high as threethirty-seconds and three sixty-fourths inch.

It will be appreciated, therefore, that the thermal treatment process ofthis invention and the resultant product provide improved barrierproperties for the protective layer as well as providing improvedadhesive bonding between the metal sheet and the protective layer. Inaddition, undesired opening characteristics Such as diaphragming andfeathering are eliminated with the result being improved openingcharacteristics for the container wall. All of this is accomplished bythe specific stress relieving, thermal post-treatment of this inventionwhich is believed to substantially completely eliminate molecularstructural dislocations established within the protective layer duringsevere mechanical working of the container wall. This is accomplishedwithout permanently effecting a meaningful alteration in thecrystallinity of the protective layer. All of this is accomplished in aneconomical fashion without requiring major investment in additionalequipment or the use of additional materials such as would be requiredin connection with repair coating practices.

Whereas particular embodiments of the invention have been describedabove for purposes of illustration, it will be evident to those skilledin the art that numerous variations of the details may be made withoutdeparting from the invention as defined in the appended claims.

We claim:

1. A container wall structure comprising a metal sheet having a scoreline defined removable sector,

an adhesive layer secured to one surface of said metal sheet,

said adhesive composed of a material selected from the group consistingof epoxy, polyester and polyurethane,

a stress relieved thermally treated continuous protec tive layer securedto said metal sheet undersurface by means of said adhesive layer,

said continuous protective layer composed of a material selected fromthe group consisting of partially crystalline polyolefin, partiallycrystalline polyolefin-ethylene copolymers, polyethylene acrylate andionomers of polyolefin,

said protective layer having a crystallinity in excess of about 50percent, and

said adhesive bond between said protective layer and said metal sheetremovable sector being of greater strength than the strength of saidprotective layer in regions underlying said score line, wherebyseverance of said score line and displacement of said removable sectorautomatically establishes severance of said underlying protective layerwithout substantial diaphragming and without feathering exceeding aboutone thirty-second inch.

2. The container wall structure of claim 1 including said protectivelayer has a thickness of about 2.5 to 3.5 mils in regions not underlyingsaid score line, and

said protective film is selected from the group consisting ofpolypropylene and a polypropyleneethylene copolymer having an ethylenecontent of about 2 to 10 percent by weight.

3. The container wall structure of claim 2 including said protectivelayer being substantially free of mi crovoids, and

said protective layer having stress relieved properties resulting fromthermal treatment at about 275 to 375 F for about 0.2 second to 4minutes and having greater barrier properties than would be present inthe protective layer in the absence of said thermal treatment.

4. The container wall structure of claim 3 including said protectivelayer having stress relieved properties resulting from thermal treatmentat about 300 to 350 F for about 0.5 to 3 minutes,

said bond between said protective layer and said metal sheet issubstantially continuous, and

said protective film is a polypropylene-ethylene copolymer having anethylene constituent of about 2 to 5 percent by weight.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,762,598 Dated October 2, 1973 Inventor(s) Herbert Gayner & David A.Smith It is certified that error appears in the above-identified patentand that said Letters Patent are hereby corrected as shown below:

Before the abstract insert "The portion of the term of the patentsubsequent to May 29, 1990 has been disclaimed.

Column 1, line 51 Delete "Pat." before Ser. NO. I

Column 1, line 53 7 Delete "Pat." before I "Ser. No.

Column 1, line 67 Delete "Pat." before "Ser. No."

Column 6, line 42 Change "information" to T --formation-- Column 6 line52 Change "posttreatment" to -post-treatment-- M Column 7, lines 4 6: 5Change "information" to formation- Column 9, line 3 Change"deiphragming" to --diaphragming- Signed and sealed this 23rd day ofJuly 197b (SEAL) Attest:

MCCOY M. GIBSON, JR. C. MARSHALL DANN Attesting Officer Commissioner ofPatents Disclaimer 3,762,598.He1"be1"t Gag new, Monroeville, and DavidA. Smith, Penn Hills, Pa. THERMALLY TREATED CONTAINER WALL. Patent datedOct. 2, 197 3. Disclaimer filed June 11, 1973, by the assignee, AluminumCompany 0 f Amem'ca. Hereby disclaims the portion of the term of thepatent subsequent to May 29, 1990.

[Ofiicz'al Gazette N ovembeq" 6, 1.973.]

2. The container wall structure of claim 1 including said protectivelayer has a thickness of about 2.5 to 3.5 mils in regions not underlyingsaid score line, and said protective film is selected from the groupconsisting of polypropylene and a polypropylene-ethylene copolymerhaving an ethylene content of abouT 2 to 10 percent by weight.
 3. Thecontainer wall structure of claim 2 including said protective layerbeing substantially free of microvoids, and said protective layer havingstress relieved properties resulting from thermal treatment at about275* to 375* F for about 0.2 second to 4 minutes and having greaterbarrier properties than would be present in the protective layer in theabsence of said thermal treatment.
 4. The container wall structure ofclaim 3 including said protective layer having stress relievedproperties resulting from thermal treatment at about 300* to 350* F forabout 0.5 to 3 minutes, said bond between said protective layer and saidmetal sheet is substantially continuous, and said protective film is apolypropylene-ethylene copolymer having an ethylene constituent of about2 to 5 percent by weight.